Vehicles require a steering system to control the direction of travel. Steering systems typically include a mechanical linkage or a mechanical connection between a steering input device (e.g. a hand wheel) and the vehicle's steerable device (e.g. road wheels). Thus, mechanical movement of the steering input device causes a corresponding mechanical movement of the steerable device. Movement of such mechanical systems is often power-assisted through the use of hydraulic or electric actuators. These device actuator systems are responsive to a detection of a driver torque and would magnify it to form the power assist torque. An actuator system that can overlay a torque to the road wheels responsive to signals other than the torque coming from the driver, is said to be capable of torque overlay functionality. The torque overlay function would be advantageous toward a pleasing driving experience. For example, the overlayed torque can be in response to a wind gust or a detection of a situation where a vehicle is losing its stability.
Similarly, an increased level of quality and comfort can be achieved with a steering system that can provide a position overlay function. Again, the position of the road wheel can be a sum of the driver's position at the hand wheel and a supplementary motion of a device responsive primarily to the hand wheel's motion. This augmentation of position is not referred to as position overlay, unless the incremental position can be responsive to inputs other than from the driver. So, to increase the stability during driving, the position of the front wheels can be controlled actively by overlaying an incremental position to that of the driver once a potential danger is detected via vehicle state sensors and/or other sensors detecting wind gust, road crowns, etc. Tunable and customer selectable variable ratio steering and driver effort can be achieved with some existing steering systems, yet these functions are less reaching capabilities since outcomes (variable effort or variable ratio steering) are directly functions of driver input torque and position.
As one achieves torque or position overlay functionalities, one could find that the other be adversely affected. For example, a position overlay system that helps with a vehicle's stability may subjectively deteriorate the torque felt by the driver and thus the torque overlay functionality. Current steering systems that are capable of active control (torque and position overlay) of the front wheels (with varying degrees of success) are “steer-by-wire” (SBW) and “active front steer” (AFS).
“Steer-by-wire” systems typically replace the mechanical linkage between the steering input device and the steerable wheels with an electrically assisted system equipped with sensors that monitor and implement the driver's intent.
For example, a position sensor will detect the displacement of the steering input device and send an electrical signal to a controller. Based upon the electrical signal, the controller activates an output device (actuator) that is attached to the vehicle's steerable device. Steerable devices include, for example, the road wheels of an automobile, the skis of a snowmobile, the nozzles or jets of a jet ski, the propellers of a boat, and the like. Thus, the controller controls the output device to adjust the position of the steerable device based upon the displacement and/or the position of the input device.
Compared to steer-by-wire systems, the mechanical linkage will have a positive effect on the acceptance of the technology from various points of view, including psychological. Therefore, a natural migration is expected from systems with mechanical linkage to steer-by-wire systems. The systems with mechanical linkage are expected to spend less energy than steer-by-wire systems since the driver provides some assist to the system and most of the road feed back is mechanically supplied back to the driver. In steer-by-wire, the system provides all of the power to position the front wheels and all of the power to provide road feel to the driver.
This migration is being made, however, to improve vehicle performance. Namely, because the input device is mechanically decoupled from the steerable device, the steer-by-wire systems eliminate undesirable feedback from the steerable device. The use of steer-by-wire systems can eliminate deleterious feedback to the driver in the form of shudders and kickback from the steerable device. For the time being, the mechanizations in a steer-by-wire system are very expensive due to backup sensors and/or backup actuation systems, and the stringent (in terms of friction, lash, accuracy, etc.) requirements for the sensors and actuator components.
Active front steer is currently designed as a system that augments the front road wheels in series with that of driver input. The AFS system maintains the mechanical link from the road wheel to the driver. Since there is a mechanical connection and it is in series with both the assist control mechanism and the driver input device, torque feedback to the driver during road wheel augmentation is a design challenge. The AFS system works in conjunction with the steering assist system such as hydraulic power steering (HPS) or electric power steering (EPS) and therefore the performance of the steering system is dependent on the performance of the active front steer system and the performance of the steering assist system. Since active front steer provides independent control of the front road wheels, the system achieves performance benefits such as stability, variable ratio, and chassis system integration.
Traditionally, steering systems are designed such that the ratio from hand wheel angle to (front) road wheel angle is fixed. This is known as the overall steering ratio. With the advent of AFS systems, the opportunity exists to dynamically modify (add or subtract) steering angle from the driver's input. For example, when steering angle is added to that of the driver's (i.e. hand wheel) angle, the road wheels are turned more (than what they would have been under fixed ratio). Thus the overall steering ratio is reduced. The consequence of these modifications, however, is a torque that's fed back to the driver (perhaps undesirable). This torque (or force) feedback can be reduced as one increases the assistive power, however, an increase in assist may not be feasible or desirable.